The present invention relates to the rapid polymerization of acrylamide copolymers. More particularly, this invention relates to the use of the rapid polymerization of water soluble acrylamide copolymers useful as photoresist compositions through the use of tetramethylamine catalytic agents in the presence of free radical initiators.
Polyacrylamide (PAM) mixed with other polymers and a copolymer of acrylamide and diacetone acrylamide (PAD) can be used in the color picture tube or photoengraving industries as a polymeric component of the light sensitive photoresist. Prior to crosslinking, the photoresist polymers are water soluble. Reciprocity-failing photoresist formulations generally consist of water soluble polymers or copolymers and photosensitizer crosslinking reagents. Such formulations may also contain a coupling agent for substrate adhesion, and a surfactant. The film is coated onto a substrate and is exposed to ultraviolet light through a mask pattern resulting in crosslinking of the polymeric components. The unexposed area is subsequently removed from the substrate with water and the pattern is formed.
Although other polymers such as polyvinylpyrrolidone (PVP) and polyvinylalcohol (PVA) have been used for this process, the acrylamide polymers and/or co-polymers are superior due to their ability to achieve higher print down and their ability to print a fine pitch high definition product with excellent edge or pattern definition.
The current state-of-the-art polymerization process for the copolymer of acrylamide and diacetone acrylamide has been described by Kohashi, et al. , Water-Soluble, reciprocity-Law-Failing Photoresist Composed of Poly(acrylamide-co-diacetone acrylamide) and Water-Soluble Bisazide, Photographic Science and Engineering, Vol. 23, No. 3, May/June (1979). Polymerization was achieved in aqueous solutions by the addition of ammonium persulfate followed by sodium sulfite as initiators to generate free radicals and then repeating the successive addition of the initiators a second time an hour and a half later to continue the free radical generation. These free radicals interact with the acrlyamide and diacetone acrylamide monomers, activating them. These activated monomers then interact with each other generating the copolymer (PAD) of acrylamide (AA) with diacetone acrylamide (DAA). This process takes between 18 and 24 hours to reach completion and the viscosity and molecular weight of the polymer are difficult to control.
To use these polymers and copolymers as a resist component in the automated production process, the polymer viscosity must fall within a narrow margin of acceptance values in order to control the photoresist thickness precisely, e.g. .+-.2 cps. Viscosity is a reflection of the molecular weight, and thus the extent of polymerization and the size of the polymer achieved during polymerization of the monomers. The co-polymer PAD is not commercially available.
The formation of high molecular weight acrylamide polymers obtained by copolymerizing acrylamide with a monovinyl monomer in the presence of a redox type catalyst comprising a persulfate, an aliphatic amine and at least one Lewis acid selected from the group consisting of chlorides and bromides of aluminum, antimony, lithium and zinc is taught in Isaokaet al., U.S. Pat. No. 3,931,122. The monovinyl monomer is preferably acrylic acid but may extend to other monovinyl monomers such as methacrylic acid, acrylonitrile and vinyl pyridine and their salts where applicable. These are considerably different from the diacetone acrylamide monomer utilized by Kohashi, et al., supra in preparing water-soluble, reciprocity-failing poly(acrylamide-co-diacetone acrylamide) photoresists. The objective of isaoka et al. was to prepare a slurry which yielded, over about a period of 8 hours, a powdered polymer product which could be subsequently used as a flocculent in removing solid fine particles from aqueous suspensions, e.g. for water purification and other environmental purposes. The Lewis acid was stated as being an essential component in the redox catalyst. When the Lewis acid was not present, an unstable slurry of a polymer formed which flocculated and could not be recovered as a powder. There is no suggestion in Isaoka et al. that an acrylamide copolymer could be obtained by rapid polymerization which would be suitable for use as a photoresist. Nor is there a suggestion that a suitable redox catalyst could be utilized which did not contain a Lewis acid. A combination of a persulfate salt and an ethanol amine or aminoethanol along with a Lewis acid are shown as the preferred redox catalysts. Other aliphatic amines such as hexamethylene diamine, diethylene triamine, triethylene tetramine, tetraethylene pentamine, polyethyleneimine, tetramethylethylene diamine, tetraethylpropylene diamine and tetramethylhexamethylene diamine are listed as possible amines to be used in combination with the persulfate salt and the Lewis acid but are not specifically exemplified.